Probing the seesaw mechanism with neutrino data and leptogenesis

Abstract

In the framework of the seesaw mechanism with three heavy right-handed Majorana neutrinos and no Higgs triplets we carry out a systematic study of the structure of the right-handed neutrino sector. Using the current low-energy neutrino data as an input and assuming hierarchical Dirac-type neutrino masses m Di , we calculate the masses M i and the mixing of the heavy neutrinos. We confront the inferred properties of these neutrinos with the constraints coming from the requirement of a successful baryogenesis via leptogenesis. In the generic case the masses of the right-handed neutrinos are highly hierarchical:Di ; the lightest mass is M 1 ≈ 10 3 − 10 6 GeV and the generated baryon-to-photon ratio η B 10 −14 is much smaller than the observed value. We find the special cases which correspond to the level crossing points, with maximal mixing between two quasi-degenerate right-handed neutrinos. Two level crossing conditions are obtained: m ee ≈ 0 (1-2 crossing) and d 12 ≈ 0 (2-3 crossing), where m ee and d 12 are respectively the 11-entry and the 12-subdeterminant of the light neutrino mass matrix in the basis where the neutrino Yukawa couplings are diagonal. We show that sufficient lepton asymmetry can be produced only in the 1-2 crossing where M 1 ≈ M 2 ≈ 10 8 GeV, M 3 ≈ 10 14 GeV and (M 2 − M 1 )/M 2 10 −5 .